Magnetic particles are used as solid phase for performing some diagnostic assays, e.g. immunoassays. Such assays comprise magnetic particles in suspension in a reaction solution contained in a vessel. When the assay is conducted, it is at some steps necessary to separate the magnetic particles from the liquid contained in the vessel.
In a known apparatus this is done by attracting the magnetic particles to the walls of the vessel by means of a magnet positioned close to the outer side wall of the vessel and by extracting the liquid from the vessel by suitable means.
In an apparatus of this kind described in U.S. Pat. No. 6,187,270 a pipetting tip is used as a vessel and a magnet is located close to a wall of the pipetting tip in order to fix a cluster of magnetic particles at a given position on the inner surface of that wall of the pipetting tip. With the cluster of magnetic particles in that position washing water is drawn into the pipetting tip for performing a washing step. This water is ejected from the pipetting tip and another portion of fresh water is drawn in. After that the pipetting tip is slowly moved away from the magnet for moving the magnetic particles towards the tip of the pipetting tip.
According to U.S. Pat. No. 6,187,270 a stationary magnet is used and an apparatus for moving the pipetting tip with respect to the magnet is used for carrying out the above described motion of the pipetting tip. A disadvantage of the latter apparatus is that the magnetic force that acts on the magnetic particles rapidly diminishes as the pipetting tip is moved away from the magnet, and the desired transport of the magnetic particles to the tip of the pipetting tip is thus relatively slow.
Another disadvantage of an apparatus of the kind described in U.S. Pat. No. 6,187,270 is that when it is desired to process in parallel liquids contained in a plurality of vessels, a corresponding plurality of transport means for moving the vessels away from the magnets are required.